Safety and arming delay devices which comprise a ball rotor having a polar moment of inertia larger than either transverse moment of inertia have been in use since their invention in the late 1800's. In such a device the ball rotor contains an explosive element which is part of the firing train. Such a firing train usually begins with the detonator or primer and ends with the main charge of the projectile. In most cases the detonator itself is contained in the ball rotor. Prior to firing the munition the rotor is held with its polar axis at some angle, such as 60.degree. to 80.degree., with the spin axis of the projectile. The explosive element contained in the ball rotor lies on the polar axis of the ball rotor thus in this condition an accidental explosion of the detonator or primer will not result in explosion of the munition. When the munition is launched, forces arising out of the difference in moments of inertia of the ball, projectile spin, and friction act in such a way as to cause the ball rotor to align its polar axis with the spin axis of the projectile. At this time the projectile is armed and an explosion of the detonator will result in an explosion of the projectile if the firing train is not broken at some other point.
Since this safety and arming device consists primarily of one moving part, its simplicity has never been equally by other safety and arming devices. For this reason a great deal of effort has been applied over the years to adapt the ball rotor to fuzing of all types of spinning projectiles. In spite of this effort, however, gear train escapements and other safety and arming delay devices have generally been used in every application except 20-30 mm. projectiles even though the cost of these safety and arming delay devices is generally two to five times higher. Ball rotor fuzing is currently being used in the 20-30 mm. projectiles because of the extreme size limitations imposed on the fuze by this projectile. Nevertheless, even in the 20-30 mm, projectiles a considerable effort has been undertaken to find a substitute for the ball rotor.
The reason ball rotor safety and arming delay devices have not found greater use in spite of the substantial research and development efforts that have been expended, lay in the wide dispersion of arming distances which have resulted and in the failure of most ball rotor systems to achieve a minimum safe arming distance. Even in the now standard M505A3 20 mm. fuze, some of the projectiles begin arming at less than 20' and all are not armed until somewhere over 100'. When these projectiles are fired from wing located guns in high performance aircraft, some of the projectiles could be armed while very close to the nose of the plane. An accidental explosion of one of these armed projectiles could seriously damage the plane and injure the pilot.
A careful mathematical analysis supported by test firings has shown that the cause of the short arming distance is partially due to high friction forces. Similarly, the cause of the wide dispersion in arming distances is due to a wide dispersion of the magnitude and location of the friction forces acting on the ball rotor. The key to successful ball rotor fuzing, therefore, lay in reducing and controlling these friction forces. This has been accomplished by this invention.
The approach taken in the Ziemba et al Pat. No. 3,397,640 granted Aug. 20, 1968 is illustrative of an attempt to solve the problems of ball rotor safety and arming devices. This patented system proved to be unacceptable because the desired reproducibility over a temperature range of -60.degree. F. to +160.degree. F. could not be obtained. The viscous liquids and silicone greases contemplated and available at that time had a variation of viscosity of 35-40 to 1 over this temperature range.